Saudi Arabia's decision to snub the United States and buy billions of dollars worth of arms from Britain may lead other oil-rich Arab states to reduce U.S. arms purchases in favor of European and communist suppliers, Western diplomats said Saturday.

Saudi Arabian press reports said the kingdom and London signed a memorandum of understanding that will ensure Britain replaces the United States as Riyadh's main weapons supplier in an estimated $12 billion deal.They said the move was taken after Riyadh lost patience with the powerful so-called Jewish lobby in Congress, which blocked the sale of sophisticated U.S. weapons to Saudi Arabia for fear it might cause escalating tension in the region and tip the balance in sophisticated weapons against Israel.

Saudi officials say they want to improve their defense capabilities not to threaten Israel, but to thwart a perceived Iranian threat to their sovereignty.

Earlier this year, the conservative but staunchly Moslem kingdom snapped diplomatic ties with Iran, accusing Tehran's ruling clergy of whipping up anti-Saudi fervor.

Kuwait hopes to buy 40 advanced F-18 jet fighters and Maverick ground-to-air missiles from the United States, but Kuwaiti officials fear the deal may be placed in jeopardy because the Senate has voted overwhelmingly against selling the Mavericks to Kuwait.

Both the Saudis and Kuwaitis have hedged their bets by purchasing arms not only from the United States but also from Western Europe, the Soviet bloc and communist China.

The secret Saudi purchase earlier this year of Chinese intercontinental ballistic missiles angered the United States and prompted Israel, in a prompt if veiled threat, to say it might bomb the missile launch sites.

Another oil-rich Persian Gulf state, Qatar, Saturday established diplomatic ties with Beijing in a move seen as a further snub to U.S. diplomacy in the region.

The Qataris are embroiled in a dispute with the Reagan administration over Qatar's reported purchase on the black market and without U.S. permission of shoulder-launched anti-aircraft missiles.

The administration blocked sales of Stingers to Qatar for fear the weapon may end up in terrorist hands. But Iran apparently found no difficulty in obtaining Stingers from the Afghan resistance - which received them free from Washington.

The Anglo-Saudi deal has come as a heavy blow to American defense contractors, and Western diplomats say other gulf states may follow suit.

"The Arab gulf states consider themselves to be reliable U.S. allies and demand to be treated as such," one envoy said.